1. Field of the Invention
The present invention is broadly concerned with improved crossflow water cooling towers and methods of use thereof wherein the towers are constructed so that entering crossflow cooling air currents exit the crossflow fill structure of the tower both laterally through the upright outlet face of the fill and upwardly through the generally horizontal upper face thereof, i.e., at least about 50% of the total air flow through the fill structure exits through the upright air outlet face while the remainder exits through the upper face; the hot water distributors for the towers are arranged adjacent the upper portion of the fill structure and are located so as to permit passage of air currents therebetween. In this way, low cooling potential air passing through the upper portion of the fill is immediately vented from the fill, so that greater volumes of more effective cooling air currents may be drawn transversely through the fill. The towers of the invention may be of the mechanical draft variety wherein a large multiple-blade fan assembly is employed in partial overlying relationship to the upper face of the fill, of the natural draft type employing a large hyperbolic stack, or of the forced mechanical draft type.
2. Description of the Prior Art
Industrial sized water cooling towers have found extensive use in large industrial, business and multiple resident complexes because of their ability to efficiently dissipate large amounts of process or occupancy generated heat to the atmosphere. Cooling towers of this type are found in various areas including factory complexes, chemical processing plants such as petrochemical facilities, near offices, at hospitals, as a part of multi-family apartments or condominiums, as a part of large commercial retail properties, warehouses and electrical generating stations including nuclear power plants.
Conventional mechanical draft crossflow cooling towers are constructed with upright unitary or sectionalized fill structure (either of the splash bar or sheet film type) surmounted by hot water distribution basins equipped with target nozzles which distribute the incoming hot water over the fill. The interior space bounded by the fill structure and the cold water basins defines a plenum for the tower. A fan assembly made up of an apertured deck, a powered fan and a surrounding venturi-shaped stack is positioned above and in communication with the plenum. In the operation of crossflow cooling towers, hot water is introduced at the top of the fill while the coldest (ambient) air is introduced along the upright sides of the tower. The highest potential for cooling exists at the top of the air inlet sides where the hottest water comes into contact with the coldest air. Once such air has been heated such that the wet bulb temperature of the air is near the water temperature, the air has no more capacity to cool the water, and such heat saturated air prevents the introduction of cooler ambient air into the fill. Air near the top of the tower typically experiences this condition because it initially contacts the hottest water, and all other water along its path of travel is about the same temperature. Air entering near the bottom of the tower initially is exposed to water that has been significantly cooled. As it traverses through the fill, the temperature of the water encountered by the bottom air currents rises, which allows the air to take on more heat.
The hot water basins in a crossflow tower are normally constructed to serve as an air seal to prevent air entering the tower through the top of the fill. Additionally, air seals along the length of the tower are provided along the inboard and outboard edges of the basins to seal from the bottom of the basins to the top of the fill. These seals prevent air from entering the spray chamber and bypassing or "short circuiting" the fill structure. Sealing of the distribution basins also minimizes the contact between incoming air currents and relatively large water particles adjacent the spray nozzles.
The plenum size in a crossflow tower generally dictates the maximum fan diameter, i.e., increasing the fan diameter necessarily requires an increase in the plenum size. Air discharge from the top of conventional crossflow fill structure enters the plenum in a generally horizontal direction, although drift eliminators may be so directed as to discharge the air in an upwardly sloping direction. In any case, the air must turn to approach the fan, which creates interference with the air flow from below which is attempting to reach the fan in a generally vertical direction. Some crossflow designs extend the plenum chamber a few feet above the hot water basin level to provide better entry of the air from the top of the fill structure into the fan. Theoretically, if this vertical extension of the plenum is of sufficient height, the sidewalls of the plenum could slope outwardly to provide for placement of a larger fan assembly. Any such expansion of the plenum would necessarily need to be gradual (about 5.degree. from the vertical) so as to manage the expansion of the air through the plenum extension without disruption. However, the gain in performance (if any) in such a tower would not be great, owing to the fact that a very great plenum extension would be required to add appreciable diameter to the fan.
An increase in the amount of air passing through the fill structure of a crossflow tower generally increases the amount of cooling. However, as the air rate increases, the pressure drop across the tower increases exponentially. Cooling towers typically operate in the range of from about 20-50 pda/sf/min (pounds of dry air/square foot/minute). Lower air rates do not produce as much cooling for a given tower as compared with higher air rates. Also, lower air rates typically result in low stack discharge velocities which may promote recirculation of effluent warm air back through the tower air inlets, reducing the efficiency of the tower. Lower air rate towers are also subject to wind disturbances which can lessen performance. On some tower designs, the pressure drop may be too large to achieve 50 pda/sf/min. air rate even with the largest fan assembly which can fit the design. Alternately, the air inlet face area may be so large that the fan-induced air flow is insufficient to reach high air rates regardless of pressure drop. For example, four-way induced draft crossflow cooling towers described in U.S. Pat. No. 4,788,013 may be subject to this problem.
It is also been proposed in the past to construct hybrid cross-counterflow cooling towers wherein crossflow fill splash bars are located in the descending water/air entrance zone of the tower. For example, UK patent no. 528,938 describes a multiple fill stack arrangement in a counterflow natural draft cooling tower. In this design, splash fill is placed at the level of the air inlet for crossflow cooling, and also above the air inlet for counterflow cooling. However, the large pressure drop incident to the fill arrangement would limit the volume of air currents exiting the upright inboard surface of the fill to no more than about 10% of the incoming air to the fill. Thus, a very high proportion of the air exiting the fill would pass through the upper horizontal surface thereof.
Other instances of combined crossflow/counterflow towers are illustrated in U.S. Pat. Nos. 3,707,277; 5,427,718; and 5,569,415. The latter patent provides an increased plenum area without increasing the width of the tower by truncating the upper inboard corners of the fill. The main problem with this idea is the complicated water distribution system required, and the difficulty in developing a reasonable air flow distribution through such a non-uniform fill bank arrangement.
Prior crossflow cooling tower designs have been limited by the perceived need to prevent "short circuiting" or upward exiting flow of air from the upper portion of the fill, i.e., to insure that essentially all of the incoming air currents pass transversely through the fill structure and exit the outlet face thereof into the plenum. This design consideration in turn significantly limits the fan size to the approximate diameter of the plenum, and thus reduces the amount of air which can be effectively drawn through the tower. All of these factors therefore contribute to less than desirable tower cooling efficiency. Moreover, hybrid crossflow/counterflow tower designs have not been successful, principally due to the fact that the different types of fill structure employed therein do not operate efficiently at the same levels of water loading; therefore, at water loadings suitable for crossflow fill, the counterflow fill sections do not perform properly, and vice versa.